From Gen VI onwards, the max Effort Value of a stat was changed from 255 to 252.
To change this, go to the `MonGainEVs` function in `src\pokemon.c`:

```c
...
if (evs[i] + (s16)evIncrease > 255)
{
    int val1 = (s16)evIncrease + 255;
    int val2 = evs[i] + evIncrease;
    evIncrease = val1 - val2;
}
...
```
And change both `255` to `252`.

That's it!